The present invention relates generally to safety devices for movable platforms, and particularly to tension sensing devices for scaffolds suspended by one or more hoist cables.
During the normal operation of a suspended scaffold, two undesirable and dangerous conditions can typically occur. The first is where the support or hoist cable is overtensioned, thereby placing an abnormal strain on the cable and increasing the probability of the cable breaking or fracturing. This strained cable condition can occur when the combined weight of the scaffold, equipment and operators exceed the rating of the cable. The strained cable condition can also arise when the upward movement of the scaffold is resisted by a fixed projection engaging a portion of the scaffold structure. The second dangerous condition is where the hoist cable is under tensioned, thereby creating an abnormal under-tension condition or lack of support from the cable and subjecting the scaffold to tipping and the drive mechanisms to misalignment. In both of these dangerous conditions, a possibility of overloading the obstruction could cause damage, or fracture and falling objects. This under-tensioned cable condition can occur when the downward movement of the scaffold is resisted by a fixed projection engaging a portion of the scaffold structure.
Prior techniques for determining the strained cable condition have typically been based upon sensing the displacement of the hoist cable relative to the scaffold structure. Since a load must be imparted to the scaffold structure in order for these prior arrangements to indicate an increase in the strain on the hoist cable, they are not capable of detecting an abnormal strain due to other types of restraining forces. For example, a force could restrain the hoist mechanism from climbing the hoist cable or restrain a mooring cable without affecting the load on the scaffold structure. Accordingly, these prior arrangements can be dangerously inaccurate and give a false sense of security to the scaffold operators.
Accordingly, it is a principal object of the present invention to provide a novel tension sensing device capable of accurately and reliably detecting both under-tensioned and strained cable conditions on the hoist cable of a suspended scaffold.
It is an additional object of the present invention to provide a tension sensing device which is capable of giving a visual indication of the tension on the hoist cable.
It is a further object of the present invention to provide a tension sensing device which is simple and rugged in construction, yet flexible in design for use in a variety of scaffold sizes and types.
It is yet another object of the present invention to provide a tension sensing device wherein the controls are grouped together at one central location so as to provide for convenient observation and the like.
It is still a further object of the present invention to provide a tension sensing device which may be provided with means for measuring the degree or angle of unlevelness of the associated scaffold structure, i.e., the present invention lends itself to being provided with instrumentation for sensing the degree of scaffold levelness.
In accordance with the foregoing objects, the present invention provides a tension sensing device generally comprising a support structure adapted to be secured to the scaffold and having guide means for directing the hoist cable through the device, lever arm means for sensing the tension on the hoist cable, and switch means for detecting the under and over-tensioned cable conditions in response to the movement of the lever arm means. The lever arm means includes a generally vertically disposed lever arm member pivotally mounted to the support structure for pivotal movement in response to a variation in the tension of the hoist cable, a sheave rotatably mounted to the lever arm member, and spring means for biasing the sheave into engagement with the hoist cable. The switch means includes a first limit switch for detecting the under-tensioned cable condition in response to a first predetermined pivotal movement of the lever arm member, and a second limit switch for detecting the over-tensioned cable condition in response to a second predetermined pivotal movement of the lower arm member in the opposite direction of the first predetermined pivotal movement. The support structure also includes a cover plate assembly with at least one section of the cover plate assembly constructed from a transparent material so that the tension on the hoist cable may also be visually observed.
Additional advantages and features of the present invention will become apparent from a reading of the detailed description of the preferred embodiments which makes reference to the following set of drawings in which: